1. Field of the Invention
The present invention pertains to an ultrasound catheter system for ablating obstructions within tubular anatomical structures such as blood vessels, and in particular, an ultrasound catheter system that is capable of crossing completely occluded obstructions in a blood vessel.
2. Description of the Prior Art
A number of ultrasound catheter systems have heretofore been proposed for use in ablating or removing obstructive material from blood vessels. These ultrasound catheter systems include an ultrasound transmission member incorporated with a catheter for transmitting ultrasound energy to the obstructive material to remove the obstructive material. The catheter is delivered percutaneously to the location of the obstruction with the aid of a guidewire. Specifically, the guidewire is first percutaneously advanced through the patient's vasculature until it reaches and crosses the region of the blood vessel where the obstructive material is located. The catheter carrying the ultrasound transmission member is then advanced along the guidewire to the location of the obstructive material. The distal tip of the catheter is caused to contact the obstructive material, and ultrasound energy is then applied to the distal tip of the catheter via the ultrasound transmission member, to remove the obstructive material. The catheter is advanced along the guidewire through the obstructive material until the distal tip of the catheter has passed through all the obstructive material. In some instances, adjunctive balloon angioplasty (PTCA) can be performed to achieve better results.
Thus, the ultrasound systems and devices described above have been effective in removing obstructive material in blood vessels when it has been possible to cross the obstructed region with a guidewire. Since the guidewire is needed to guide the distal tip of an ultrasound catheter through the obstructed region, these ultrasound catheter systems are ineffective in treating a blood vessel which has been completely occluded by obstructive material, where the guidewire is not able to cross the obstructive material at the obstructed region.
Completely occluded obstructions are also troublesome since the number of treatment options are limited. For example, PTCA or balloon angioplasty procedures cannot be carried out to ablate completely occluded obstructions because it is not possible to place an expandable balloon inside the obstructed region. In addition, ultrasound catheter systems, such as those described above, are ineffective for the reasons explained above. Of the remaining options, highly invasive alternatives such as cardiovascular surgery are possible, but are undesirable.
There are also available less invasive techniques such as laser techniques and cutting techniques such as atherectomy and thrombectomy where rotational cutters, shavers, and coring means are used to cut through the occlusion. However, not only do these less invasive techniques also require that a guidewire be used to cross the obstructed region, but they also have additional drawbacks. For example, the cutting techniques require the use of cutting devices that are often bulky in nature, and therefore cannot be used, without a guidewire, to cut obstructive material. In addition, there is always a possibility that some of these cutting devices may damage or even perforate the blood vessel wall if they are not used with guidewire support. Further, the laser techniques, when used without guidewire support, may perforate the vessel wall, and can cause mechanical thermal damage or necrosis, which kills healthy cells and nerves. Laser systems for providing the laser energy can also be expensive.
One attempt to address this problem is illustrated in U.S. Pat. No. 5,427,118 to Nita et al., in which an ultrasonic guidewire is provided for crossing completely occluded blood vessels. The ultrasonic guidewire creates the necessary pathway through the obstructive material, and a balloon catheter is then advanced inside the obstructive material and the balloon expanded to dilate the obstructive material. Unfortunately, the ultrasonic guidewire in U.S. Pat. No. 5,427,118 is not by itself fully effective in removing the obstructive material because the low profile throughout the length of the guidewire means that an insufficient amount of ultrasound energy is transmitted from the transducer to the distal end for treating the occlusion.
Thus, there still exists a need for improved ultrasound systems having ultrasound catheters that can treat and remove obstructive material from fully occluded blood vessels.